An antenna is generally a transducer device that receives or transmits electromagnetic radiation. The frequency of radiation that may be received or transmitted by an antenna is dependent on the size of the antenna, and the speed of light and the distance that electrons can travel (electron mobility) in the material medium of the antenna. Because electromagnetic waves propagate more slowly in a medium than in free space, the same number of waves will span a greater distance in free space than in the transmission medium, hence the transmission medium is said to have an electrical length that is greater than its physical length. Typically, the electrical length of an antenna is expressed in units of the wavelength (in the antenna medium) corresponding to the resonant frequency of the antenna.
Antennas are typically associated with signals having frequency of about 30 kilohertz (kHz) to about 30 gigahertz (GHz), and may be associated with, for example, longwave AM radio broadcasting, wireless LAN, radars and satellite television broadcasting. In general, the electrical length of an antenna is on the order of the free-space wavelength of the radiation at which the antenna is resonant. For example, a dipole antenna is typically about ¼th the free-space wavelength. Similarly, the physical length of an antenna is on the order of the wavelength in the antenna medium of the radiation at which the antenna is resonant. Given that the wavelength of electromagnetic radiation is shorter in a medium than in free space, the physical length of an antenna is typically shorter than its electrical length.
The electrical length of an antenna can be modified in order to cause an antenna of a particular physical length to resonate at a required frequency, by adding suitable reactive elements such as capacitors or inductors. For low frequency (long wavelength) transmission or reception, the physical length of an antenna becomes too long to be economically or physically viable. The electrical length of the antenna is, therefore, lengthened by adding suitable reactive elements to allow a shorter antenna to resonate at lower frequencies. On the other hand transmission or reception of higher frequencies, for example optical frequencies, may necessitate shortening the electrical length of the antenna in order to make the physical length longer.